In small outboard marine drives, with the aim of simplifying the structure and reducing the weight, the power output from the engine is directly transmitted to the propeller via a drive shaft. However, when it is desired to propel the boat or other watercraft at a relatively low speed, for instance for troll fishing, it is necessary to install a clutch in the path of power transmission from the engine to the propeller. Japanese UM publication (kokoku) No. 60-24714 discloses a small outboard marine drive incorporated with a centrifugal clutch which is interposed between a lower end of a crankshaft of a vertically oriented engine and an upper end of a drive shaft. The centrifugal clutch engages when the rotational speed of the engine is higher than a certain level, and disengages when the rotational speed drops below this level.
However, some consumers may wish to have such a centrifugal clutch, but others may not need it. Elimination of a centrifugal clutch will simplify the structure of the outboard marine drive, and make it lighter in weight and more economical. Also, from the manufacturer's view point, adapting the production facilities to two different models increases the efforts required for production management and the number and variety of component parts that need to be stocked. The distributor also suffers because he needs to keep two models in stock.
Based on such considerations, the inventors have realized that, to meet the need of a widest range of consumers, it is desirable for the outboard marine drive to be readily converted between a model equipped with a centrifugal clutch and another model not equipped with a centrifugal clutch.